1. Field of the Invention
The present invention relates to a receiving apparatus in a code spreading system, and, more particularly, to a receiving apparatus that has as few circuit portions as possible which process a reception signal of a high frequency band.
2. Description of the Related Art
CDMA (Code Division Multiple Access), one of spread spectrum communication systems, is receiving an attention as a communication system which can produce a greater number of channels within a limited frequency in portable telephone communication, fast data communication and so forth. In this CDMA communication system, communication between two communication units is carried out with a transmission signal spread with a specific spreading code in the frequency band of the communication channel. As a result, a signal to be communicated is in the same frequency band as other signals to be communicated, and is separated from them only by the specific spreading code. That is, a transmitted signal is subjected to spread demodulation (despreading) with a spreading code on the reception side, thereby obtaining the original transmission signal from the communication channel. The transmission signal spread with the spreading code is subjected to digital modulation such as quadrature phase shift keying and is then transmitted. The digital modulation like quadrature phase shift keying can make the frequency band of the transmission wave as narrow as possible and can reduce interference with other transmission waves on the same communication channel.
Because the code spreading process, quadrature phase shift keying and the like are all carried out with digital signals, recently, those processes are generally executed by a digital signal processor.
FIG. 7 is a diagram exemplifying a conventional receiving apparatus of a code spread communication type. In this prior art, a communication signal received at a reception antenna 1 is passed through a band-pass filter 2 to yield a signal of the frequency band of the communication channel, which is in turn amplified by a low-noise amplifier 3. A multiplier 4 multiplies the amplified signal by a signal Lo1 of a local frequency f.sub.L so that the carrier frequency f.sub.R is dropped to an intermediate frequency f.sub.M. As a result, the frequency becomes f.sub.R -f.sub.L and f.sub.R +f.sub.L, and the resultant signal is then passed through a band-pass filter 5 to yield a signal 6 of the intermediate frequency f.sub.M =f.sub.R -f.sub.L.
Then, multipliers 7 and 8 multiply this signal by a signal whose frequency is a second local frequency Lo2 shifted by .pi./2, thereby accomplishing orthogonal detection (orthogonal demodulation). Specifically, a transmission signal, which has undergone orthogonal modulation on the transmission side, is multiplied by a signal having the second local frequency Lo2 shifted by .pi./2, thus yielding an I component signal and a Q component signal, both of intermediate frequencies. Those two component signals pass through the respective low-pass filters and are converted to digital signals by AD (Analog-to-Digital) converters 56 and 58, respectively. The digital signals are then input to a digital signal processor 70. In the digital signal processor 70, a code spread band-band signal is subjected to spread demodulation or despreading (code correlation) with a spreading code to return to the original digital signal.
According to this prior art, however, spread demodulation using a spreading code is carried out in the digital signal processor 70 at the last stage, so that the circuit up to the digital signal processor 70 should process signals of a dynamic range which include signals of other channels of the same frequency band. Particularly, the greater the multiplicity becomes by increasing the number of channels, the wider the required dynamic range and the band width of signals become. It is thus necessary to secure a sufficiently wide dynamic range for the analog circuit extending from the reception antenna 1 to the digital signal processor 70. This demands a large SN ratio.
Further, spread demodulation or despreading (code correlation) with a spreading code requires fast digital processing. In particularly, as the multiplicity gets higher, the demand for this fast digital processing is further intensified, which results in increased power dissipation of the digital signal processor 70. This is contradictory to the demand for portable communications terminals or the like to reduce power consumption due to which the operation speed of the digital signal processor cannot be made sufficiently fast.